One of the most intriguing areas of forest therapy research is the relationship between phytoncides and immune function.
Phytoncides are natural compounds produced by plants and trees. They are part of the plant world’s own defence system, helping protect against bacteria, fungi, insects and other threats. When humans spend time in forests, we may inhale small amounts of these compounds through the forest air.
In 2024, Lew and Fleming published a systematic review and meta-analysis examining phytoncides and immune functioning across forest and non-forest settings.
Promising, but still emerging
The authors reported favourable immune-related outcomes, including changes in natural killer cell activation. They also rated the certainty of evidence as low and called for more rigorous randomised controlled trials.
This distinction matters. The results are exciting, but the field is still young. We should not claim that forest therapy prevents or treats disease. What we can say is that research is exploring plausible biological pathways through which forest environments may support immune-related processes.
For participants, the practical takeaway is simple: regular time in forest settings may offer more than emotional calm. It may also provide subtle physiological stimulation that science is only beginning to understand.
Forest therapy invites us to experience that possibility through slow, safe and guided immersion.
Further reading: Lew and Fleming 2024 review.